Tone control circuit



TONE CONTROL ClRCUIT Filed Nov. 16, 1962 TONE ARM ECQTION United States Patent h ice 3,332,041 Patented July 18, 1967 3,332,041 GRNE CONTROL CIRCUIT George A. Wilson and Hammond B. Dorsey, Baltimore, Mtl., assignors to The Bendix Corporation, Baltimore, Md., a corporation of Delaware Filed Nov. 16, 1962, Ser. No. 238,197 2 Ciaims. (Cl. 333-28) ABSTRACT 0F THE DISCLOSURE A tone control circuit for an audio amplifiers utilizing an adjustable resistance bass boost voltage divider and a single treble cut capacitor generally parallel counectable to a tapped volume control potentiometer. The potentiometer tap and the divider center tap may be resistance coupled.

This invention relates to a tone control circuit for an audio amplifier.

In even the least expensive automobile radio receivers a tone control is usually required because `of the need to contend with greatly varying amounts and kinds of background noise. It also permits the listener to suit his own preference as to response characteristics and provides some compensation for speaker and batlie limitations. A typical satisfactory tone control circuit provides for both emphasis at the lower frequency end of the audio spectrum (bass boost) and deemphasis at the higher frequency end (treble cut). When used with a typical potentiometer type of control, the normal operation of the receiver (without any effect from the tone control) usually is provided when the potentiometer is set at one end of its travel. The amount of bass boost is increased as the tone control is moved toward the center, reaching a maximum at the center. Further movement in the same direction results in introducing a treble cut which effeet reaches a maximum at the opposite end of travel of the tone control potentiometer. This type of response normally requires a resistance-capacitance network including separate capacitance means in each of two separate branches to accomplish the desired bass boost and treble cut. Such circuits do add an appreciable element of cost to the receiver and so it is an object of the present invention to provide a tone control circuit which will accomplish the desired frequency discrimination and which is less expensive to produce than those presently in the field.

It is another object of the present invention to provide a tone control circuit which accomplishes the above object and in which the bass boost tends to be `a substantially linear function over the entire range of the tone control.

It is a further object of the present invention to provide a tone control circuit accomplishing the above objects and in which the direct current impedance change is minimized when the tone control is moved to different positions.

Other objects and advantages will become apparent from the following specification taken in connection with the accompanying drawings in which:

FIG. 1 is a schematic drawing of a tone control circuit embodying our invention;

FIG. 2 is a graph showing the frequency discrimination characteristics of the circuit of FIG. l;

FIG. 3 is a schematic drawing of a modified form of our invention in which a step function switch is used rather than a tone control potentiometer; and

FIG. 4 is a schematic drawing of an additional modification of our invention using a step function switch.

Referring now to FIG. l, an input terminal is shown at numeral 10 and a common terminal at numeral 12. This common terminal may be at ground potential or it may be at a higher D.C. potential such as would be typical in transistor circuitry. In this latter case, the common line might typically be at a value of about 12 volts D.C. (filtered). Hereafter, any connection to a line at this potential will be referred to as to common The signal is supplied between terminals 10 and 12 across the volume control potentiometer 14 which has a tap 16 about halfway across and a slider 18, such that the output may be connected to succeeding stages between slider 18 and terminal 12. The location of tap 16 may be selected to afford a desired `amount of bass boost. Also connected between terminals 10 and 12, in parallel with potentiometer 14, is a tone control potentiometer 20 having a slider 22. A resistor 24 is connected between slider 22 and the volume control tap 16. Connected to a junction 26 between the slider 22 and resistor 24 and the common line is a capacitor 28.

When the tone control slider 22 is at the lower end of the potentiometer 20, the capacitor 28 is effectively short circuited and the tone control is ineffective. As the slider is moved upwardly along the potentiometer 20, the proportionate amount of signal voltage drop across capacitor 28 increases, thus causing proportionally greater bass boost and increasing treble cut. With the slider 22 at the top of potentiometer 20 the bass boost is at a maximum and the treble is attenuated to the maximum extent. This characteristic is shown in the graph (FIG. 2) wherein decibels gain or loss from 1000 c.p.s. at the normal position is plotted vertically vs. frequency, the latter being plotted on a logarithmic scale horizontally. The solid curve shows the response characteristic of a typical automobile broadcast receiver without any tone control action (slider 22 at the bottom of the tone control potentiometer). It is apparent that there is some attenuation at the low frequency end of the spectrum, followed by a slight boost in the mid-range and a significant attenuation with increasing frequencies above about 1000 c.p.s. With the slider 22 moved to its middle position (50%) the amount of boost at the lower frequencies is considerably greater than for the normal condition and the attenuation at the frequencies above about 600 c.p.s. is slight, as shown in the dotted line. Moving the slider 22 to the top of potentiometer 20 results in a characteristic such as that shown in the dashed line, wherein the boost of signals below about 600 c.p.s. is very substantial and the attenuation of the treble above about 600 c.p.s. is even greater. A typical prior art tone control circuit with at least two capacitors in two separate branches would provide another bend in the response curve at the high frequency end, thus substantially reducing the rate of attenuation at these frequencies, but with the amount of bass boost and treble cut usually required this ability is not significantly useful for automobile radios. The characteristic shown in FIG. 2 is entirely satisfactory for this purpose and is made possible by means of our circuit which provides both bass boost and treble cut in substantial amounts with a single capacitor. Where really substantial production runs are encountered, this simplified Itone control circuit provides a very worthwhile saving in production cost while providing all the necessary functions of the conventional circuits. The substantial linearity in the amount of bass boost afforded over the range of the tone -control potentiometer provides another significant advantage. It is also Iapparent that the D.C. impedance change with setting changes is minimized because the effective amount of resistance in the circuit does not vary greatly.

A modified form of our tone control circuit is shown in FIG. 3. This circuit is essentially the same as that of FIG. 1, except that it is -modi1ied to Vuse a step control switch. In this circuit a volume control potentiometer 30 is connected between an input terminal 32 and a common voltage line 34. The output of the circuit is taken between the common line 34 and the slider 36 of potentiometer 30. A resistor 38 is connected to a tap 40 on potentiometer 30 and at its opposite end has connections with a capacitor 42 and the movable contact 44 of a multiposition switch 46. Stationary contact number 1 of switch 46 is directly connected to the common line 34, stationary contact number 2is open, stationary contact number 3 is connected to the input terminal 32 through a resistor 48, and stationary contact 4 is connected directly to the input terminal 32.

When the movable switch member 44 is on contact number 1, as shown, the capacitor 42 is essentially short circuited and the circuit is basically that when the slider 22 of FIG. 1 is atfthe bottom. The tone control circuit is ineffective. With member 44 on contact 2, the bottom half of the potentiometer 30is shunted by resistor 38 and capacitor 42` and this provides a partial bass boost position with vlittle treble cut. When member 44 engages contact 3, the circuit is essentially that of FIG. l as illustrated and provides partial bass boost and partial ,treble cut. When member 44 engages contact v4, the circuit is essentially the same as that` of FIG. 1 when the slider 22 vis at the top of potentiometer 20, thus providing full bass boost and full treble cut.

A slightly different step control arrangement is shown in FIG. 4. In this circuit the volume control potentiometer 50 is connectedbetween input terminal 52 and the common voltage line 54. The output appears between the slider 56 of potentiometer 50 and the common line 54..A multiposition switch 58 having two shorting sections 60 and 62 provides selectable connections among a plurality of stationary contacts 1 through 6. Contact 1 is connected to common, tap 64 of potentiometer 50 through a resistor 66, contact 4 is connected to common through a capacitor 68, contact is connected to the input terminal through `a resistor 70 and contact 6 is directly connected to the input terminal.

The normal position is as shown. Terminals 1 and 3 are connectedthrough shorting section 62 and this con nects resistor 66 to common and leaves capacitor 68 disconnected. A partial bass boost position is afforded when. switch 58 isrotated counterclockwise onenotch, thus causing shorting section 62 to connect contacts 2, 3 and 4. This produces the same circuit as that produced in FIG. 3 when member 44 engages contact-2. Rotation of switch 58 another notch counterclockwise causes the shorting section 62 to effectively connect resistor 66,

capacitor 68 and resistor 70 to a common junction, thus producing an arrangement like that in FIG. 3 when movable contact 44 engages contact 3. This gives full bass boos and partial treble cut. A fourth position is produced by rotating the switch 58 an additional notch `which causes shorting section 62 to connect contacts 4, J5 and 6 contact 2 is open, contact 3 is connected to a together and shorting section 64 to connect contacts 1 and 3 together. This provides an arrangement where the lower sisting of an input terminal, a tapped volume control f potentiometer connected between said input terminal and a common terminal, a resistor having one side connected f to said tap, and capacitance means connected between the opposite side of said rresistor and common:

tone selecting means including resistance means connectable in parallel with said volume control potentiometer between said input and common, and switching means capable of selecting from among a plurality of positions connected to the junction between said resistor and said capacitance means which, when in a rst position provides a short circuit connection across said capacitance means, when in a second position disconnects said resistancenmeans fromsaid circuit, when in a third position connects said resistance means across at least part of said potentiometer, and when in a fourth position connectssaid junction with said input terminal. 2. A tone control circuit `for an audio amplifier consisting of an input terminal and a common terminal, va

tapped volume control potentiometer connected between.

said input terminal and said common terminal, a resistor having one side connected to said tap, capacitance means having one side connected to. common and resistance means having one side connected to said input terminal: tone selecting `means including multiposition switching means which, when in a first position connects said resistor to common, when in a second position connects the opposite sides of said resistor and capacitance means together, when in a third position connects the opposite sides of said resistor, said resistance means and said capacitance means together, and when in a fourth position connects the opposite side of said capacitance means to said input terminal and the opposite side of said resistor to said common terminal.

References Cited 1 UNITED STATES PATENTS 1,914,629 6/1933 Aguirri 333-28 1,938,256 12/1933 Jacobs 333-28 2,069,853 2/ 1937 Schade 333-28 2,212,389 8/ 1940 Chorpening 179-100.4 2,812,498 11/1957 Hall 333--28 HERMAN KARL SAALBACH, Primary Examiner. C. BARAFF, Assistant Examiner. 

1. A TONE CONTROL CIRCUIT FOR AN AUDIO AMPLIFIER CONSISTING OF AN INPUT TERMINAL, A TAPPED VOLUME CONTROL POTENTIOMETER CONNECTED BETWEEN SAID INPUT TERMINAL AND A COMMON TERMINAL, A RESISTOR HAVING ONE SIDE CONNECTED TO SAID TAP, AND CAPACITANCE MEANS CONNECTED BETWEEN THE OPPOSITE SIDE OF SAID RESISTOR AND COMMON: TONE SELECTING MEANS INCLUDING RESISTANCE MEANS CONNECTABLE IN PARALLEL WITH SAID VOLUME CONTROL POTENTIOMETER BETWEEN SAID INPUT AND COMMON, AND SWITCHING MEANS CAPABLE OF SELECTING FROM AMONG A PLURALITY OF POSITIONS CONNECTED TO THE JUNCTION BETWEEN SAID RESISTOR AND SAID CAPACITANCE MEANS WHICH, WHEN IN A FIRST POSITION PROVIDES A SHORT CIRCUIT CONNECTION ACROSS SAID CAPACITANCE MEANS, WHEN IN A SECOND POSITION DISCONNECTS SAID RESISTANCE MEANS FROM SAID CIRCUIT, WHEN IN A THIRD POSITION CONNECTS SAID RESISTANCE MEANS ACROSS AT LEAST PART OF SAID POTENTIOMETER, AND WHEN IN A FOURTH POSITION CONNECTS SAID JUNCTION WITH SAID INPUT TERMINAL. 